'Model' and 'ViewModel' in Knockout.js

In a JavaScript-implemented MVVM pattern, often the "Model" part is supplied by the web api. The data that is provided to the page is the model. Now whether that data is contained in an separate model object once it's in JavaScript - that's another story.

Often a View Model is just a Model that has been augmented with properties and functions to support the particular view that it is applied to. Client-side calculations, drop-down look-up values, client-side validation routines, etc. In this case, the view model might look like this:

var vm = {
    save: function(){ save logic... },
    cancel: function(){ cancel logic... },
    states: ko.observable(), //list of states for state dropdown
    customerId: ko.observable(),
    customerFirstName: ko.observable(),
    customerLastName: ko.observable()
};

Other times, the model will be maintained in a separate object. In that case the view model might look like this:

var customerModel = getCustomerFromDataSource();
var vm = {
    save: function(){ save logic... },
    cancel: function(){ cancel logic... },
    states: ko.observable(), //list of states for state dropdown
    customer: customerModel
};

The main thing to keep in mind is that the Model is the data and the View Model is the layer that makes your Model available to the view (usually via data-binding). Sometimes the Model may be a separate class; other times the Model is just a known set of properties in the View Model.

Model
Models hold information, but typically don’t handle behaviour

View
View contains the data-bindings, events and behaviours which require an understanding of the Model and ViewModel. Although these behaviours can be mapped to properties, the View is still responsible for handling events to the ViewModel

ViewModel
ViewModel sits behind our UI layer. It exposes data needed by a View (from a Model) and can be viewed as the source our Views go to for both data and actions.

You can find out more information in the following link here
You can also find more information about mvc and mvvm in stackoverflow question